Fix decoding unterminated varint

[yihuang]

When decoding the prefix varint encoded length, no check on the unterminated variable length encoded length.
After the pr to integer-encoding-rs merged and released, change the dependency.

[yihuang]

After reading the abci protocol doc, why it's different from out implementation:

The first byte in the prefix represents the length of the Big Endian encoded length. The remaining bytes in the prefix are the Big Endian encoded length.

For example, if the proto3 encoded ABCI message is 0xDEADBEEF (4 bytes), the length-prefixed message is
0x0104DEADBEEF. If the proto3 encoded ABCI message is 65535 bytes long, the length-prefixed message would be like
0x02FFFF....

It seems we don't handle the first byte?

[tomtau]

will wait for dermesser/integer-encoding-rs#10